1. Field of the Invention
The present invention relates to a temperature detecting device for a heated rotary member, adapted for use in an image fixing device of an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus.
2. Related Background Art
An image fixing device, employing a heating roller heated by a heater, is widely used in image forming apparatus such as a copying machine or a laser beam printer.
FIG. 5 is a cross-sectional view of an example of the thermal fixing device, wherein provided are a heating roller 1 with a therein; a pressure roller 2; a temperature detecting device 8 for detecting the temperature of said heating roller 1; and a plate spring for maintaining the temperature detecting device in contact with the heating roller 1.
FIG. 4 is a magnified view of the heating and pressure rollers and the temperature detecting device, wherein, on an electric circuit board 5, there is adhered rubber sponge 4 in which a thermistor is embedded and which is covered by an insulation tape 7, wound in the circumferential direction of the heating roller. Such device, however, may show significant deformation of the sponge 4 as shown in FIG. 6, whereby the temperature detecting element 3, such as a thermistor, is displaced from the designed position. Consequently, the detecting element 3 may lose contact with the heating roller 1 or may contact said roller with a pressure lower than the designed value, thereby making precise temperature detection difficult.
Recently, it is considered to apply a bias voltage to the heating roller, in order to prevent toner offsetting in electrostatic manner. It is however difficult to maintain a sufficient circumferential distance between the surfacial position of the heating roller where said bias voltage is applied and the thermistor.